feat: graduate to-questionnaire into the productivity bucket

Move `to-questionnaire` out of in-progress into `productivity/` and wire
it up as a promoted skill: plugin.json entry, top-level + Productivity
READMEs under User-invoked, a docs page at
docs/productivity/to-questionnaire.md, and a Standalone route in
ask-matt framing it as the inverse of /grill-me.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

.claude-plugin/plugin.json

@@ -40,6 +40,7 @@
     "./skills/productivity/grilling",
     "./skills/productivity/handoff",
     "./skills/productivity/teach",
+    "./skills/productivity/to-questionnaire",
     "./skills/productivity/writing-for-agents"
   ]
 }